Presently, DCA chips are bonded to metallized contacts (DCA sites) arranged on substrates such as printed circuit boards (PCB's) by applying molten solder particles to a substrate, flattening the particles, applying sticky flux to the substrate, placing components (DCA chips) onto the flattened particles and reflowing the joints.
IBM Research Disclosure 27109 describes a tool for placing solid solder balls on a ceramic surface solder device. The balls are of the order of 0.625 mm diameter. The ceramic surface mount device is screen printed with sticky solder paste only at contact sites and the device positioned above a template having an arrangement of apertures containing balls positioned to correspond to the sticky solder paste sites. The balls are held in place on the template by gravity with assistance from a vacuum, which is released to allow the sticky solder paste on the surface mount device to extract the balls from the template. The use of a template with a vacuum also aids the installation of the balls onto the template as the balls are simply spilled over and fall into place on the template.
DCA sites have contacts at a very fine pitch, so it is critical that an exact volume of solder is deposited onto the DCA sites to form a joint. Too little solder will result in an open circuit, while too much solder will result in a short circuit. Another problem is that solder pastes cannot be readily printed to such a fine pitch, as clogging of the apertures used for printing results. In addition, paste registration to DCA sites over a whole substrate would be impossible.
A repair of an open circuit or a short circuit contact for this technology is difficult so a method that can guarantee accurate deposition volume to minimize repair activity is desired. Deposition of molten solder requires an inner and outer mask to be accurately aligned to each other, and then the combination aligned to the DCA site. The alignment of the inner and outer masks is both difficult and time consuming with no guarantee that the finished mask will give the required volumes on all DCA sites. Other variable parameters which must be precisely controlled include the thickness of inner and outer masks used to deposit the molten solder, hole size of the inner mask, stand off height of the outer mask and the air pressure which forces molten solder through the masks.